Humidity sensors are widely used in the monitoring and control of industrial processes, environmental applications, electronic and biotechnology sectors, agriculture, libraries and household applications. In the last few years an increasing demand has developed for low cost humidity sensors with high accuracy, good reproducibility and long-term stability. Unfortunately, good humidity sensors are very expensive and most of the inexpensive ones do not perform well at above 70 percent or below 20 percent relative humidity.
Capacitive humidity sensors have been constructed by sandwiching a humidity-sensitive material between two parallel electrodes. Sulfonated fluoropolymers have been used as the humidity-sensitive material because of their excellent thermal and mechanical stability, and the capability of extremely fast and accurate response to change in humidity.
One of the most widely used sulfonated fluoropolymers in construction of humidity sensors include copolymers of CF2═CFOCF2CF(CF3)OCF2CF2SO3H and tetrafluoroethylene. Polymers of this type are available under the trade designation NAFION from E. I. du Pont de Nemours and Company of Wilmington, Del. For example, U.S. Pat. No. 4,662,220 (Laue) reports a method and apparatus using a known water-absorbent polymer as a capacitor which is operated at a DC voltage for measuring relative humidity. When formed as a layer between porous electrically-conductive electrodes and operated in an RC oscillator circuit, the oscillator frequency varies inversely with the partial pressure of the moisture to be measured. In a preferred embodiment, the capacitor is formed from a NAFION polymer.
U.S. Pat. No. 5,036,704 (Pusatcioglu et al.) reports a moisture sensor that utilizes a sulfonated fluorocarbon film having a thickness of less than about one micron and capable of providing accurate measurements of humidity and extremely fast response to change in humidity. A preferred embodiment of an electrical system is disclosed for use with the sensor, and a sulfonated tetrafluoroethylene perfluoroether copolymer form of the film is especially preferred.